1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly, to a method of providing a highly reliable oxide film whose interface state on an interface between a silicon carbide (SiC) substrate and an oxide film is reduced in an oxidation step on the SiC substrate.
2. Description of the Background Art
Silicon carbide (SiC) has a dielectric breakdown field strength higher by approximately one order than that of silicon (Si), which makes it possible to reduce a thickness of a drift layer thereof for keeping a breakdown voltage to one-tenth of a drift layer of Si. This enables a loss reduction in power devices.
However, between a silicon surface and a carbon surface of SiC, there are exhibited different physical properties on surfaces and interfaces due to differences in ionicity of crystal and bond sequence, and thus a difference occurs in thermal oxidation rate when oxidation is performed on the SiC substrate. Therefore, when forming a gate oxide film in a SiC-metal oxide semiconductor field-effect transistor (MOSFET), oxidation conditions need to be optimized depending on a crystal plane. This is because a difference in oxidation conditions considerably affects the mobility of an inversion channel and the reliability of an oxide film.
Conventional oxidation techniques are classified into two types including the dry oxidation technique of supplying only oxygen to form an oxide film, and the wet oxidation technique of burning oxygen and hydrogen in front of a reactor and supplying a water-vapor atmosphere thereof to the reactor to form an oxide film.
In the gate oxide film formed on the SiC substrate by the wet oxidation technique, hydrogen atoms in a water-vapor atmosphere effectively hydrogen-terminate dangling bond on the interface, which produces an effect of reducing an interface state. Accordingly, the inversion channel mobility of the MOSFET is improved. Further, in the wet oxidation technique, strong oxidation power tends to promote desorption of the residual carbon on the interface, to thereby reduce an interface state. For this reason, wet oxidation is desirably employed.
When the reliability of an oxide film by the wet oxidation technique is checked, however, the dielectric breakdown field strength is inferior to a gate oxide film formed by the dry oxidation technique due to a large amount of hydroxy groups (OH groups) contained in the gate oxide film.
Therefore, in “SiC-MOSFET with High Channel Mobility and High Reliability”, Toshiba Review, Vol. 63, No. 10, 2008, it is devised that in forming a gate oxide film of a SiC-MOSFET, oxidation is performed by the dry oxidation technique, and then oxidation is performed again by the wet oxidation technique so as to achieve high mobility as well as high reliability.
However, in the method described in “SiC-MOSFET with High Channel Mobility and High Reliability”, hydroxy groups are inevitably contained due to wet oxidation, which requires the method of forming a gate oxide film capable of achieving higher reliability.